Phosphorylation and dephosphorylation of proteins are thought to be one mechanism for regulating various cell functions, including cell growth. We have been studying the potential role that phosphorylation of myosin heavy chains (MHCs) and/or myosin light chains (MLCs) may play in growth and differentiation of smooth muscle cells, using the primary culture cells derived from rat aorta. MHCs from smooth muscle myosin were found to be phosphorylated at serine and threonine residues. The stoichiometry of phosphorylation is approximately 1 mole of phosphate/mole of MHC. Phosphorylation of MHC were also observed in intact rat aorta, preferentially at serine residues. MLCs were also phosphorylated in cultured smooth muscle cells, in the presence or absence of mitogens. Myosin light chain kinase is responsible for this phosphorylation and gives two different forms of phosphorylated MLC: one is singly phosphorylated (at serine residue) and the other is doubly phosphorylated (at serine and threonine residues). Treatment of the cells with dibutyryl cAMP caused a decrease in the level of MLC phosphorylation, especially of the doubly phosphorylated form, concomitant with cellular shape change. The physiological meaning of these phosphorylations is under study. We have been investigating the effects of some growth factors and vasoactive factors on the expression of myosin heavy chain isoforms at the protein level. Insulin, platelet-derived growth factor as well as fetal calf serum caused an increase of synthesis (35S-methionine incorporation) of nonmuscle myosin. On the other hand, platelet poor plasma from adult human sources caused an increase of synthesis of smooth muscle myosin in primary culture of rat aorta.